nr0: dry-run reuses real paging/redirect logic via shared helpers

collect_dry_run_page_plan re-derived page_count via the ceil formula and
redirect_count via a magic "*4+2", duplicating logic owned by
album_page_records (pagination) and render_page_view_redirects (redirect
files). The preview could silently drift from a real --generate.

Single source of truth:
- album_page_count_for_image_count (album-photo-select.source.sh) owns the
  MAXPREVIEWS-per-page grouping count that album_page_records realises.
- ALBUM_REDIRECTS_PER_PAGE=4 / ALBUM_REDIRECTS_LAST_PAGE_EXTRA=2 +
  album_redirect_count_for_page_count (album-render.source.sh) own the
  per-page (4) plus last-page-extra (2) redirect tally that
  render_page_view_redirects actually emits.

dry-run now predicts both counts through these helpers (no dist files
touched, side-effect free). Confirmed the real redirect count is
page_count*4+2, so output is byte-identical: partial-final-page (3 preview
pages / 14 navigation redirects) and empty album (0 / 0) unchanged.

Regenerated bin/shuriken; just test/shellcheck/check-generated all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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